Wireless communication networks have looked to high gain antennas, such as phase array antennas, to realize large gains and scan angles in order to compensate adverse channel conditions and high pathloss during high frequency band transmissions. Directional beamforming can be used to boost signal power for both transmission (TX) and reception (RX) ends of a wireless communication system. As the gain of an antenna (or antenna array) is inversely proportional to the beamwidth of the beam pattern formed by the antenna, the antenna needs to reduce the beamwidth to achieve high beamforming gain to alleviate performance degradations caused by, for example, pathloss. As a result, the network and/or user equipment need to perform beam alignment, which requires extra steps to align the direction of the beam(s) toward the target for transmission and/or reception. To achieve higher beamforming gains, the network needs to spend more resources on beam alignment as the beamwidth gets increasingly narrower.
A network and a UE may apply different kinds of beams based on different performance requirements and their respective measurements. Also, reference signaling may be applied for radio resource management (RRM) purposes. A coarse beam has a wider beamwidth but less beamforming gain, as compared to a fine beam that has a much narrower beamwidth but more beamforming gain. For example, a coarse beam with a wide beamwidth is more useful for carrying control information when a UE attempts to access a cell. Thus, coarse beams are more suitable during an initial access phase, as wide beamwidths have better chances to establish a connection with a cell (e.g., due to large coverage and simple synchronization). On the other hand, fine beams are more suitable for high speed data transmissions at high frequencies and with high gains.
Although a coarse beam with a wide beamwidth may be beneficial during the initial access phase, factors such as low beamforming gain of the coarse beam may impact the performance. For example, a wide beamwidth may cause excessive interference during a random access channel (RACH) procedure. Lower beamforming gain also means that it is hard for UEs at the cell boundary or moving across the boundary of a source cell and a target cell to establish a connection with the target cell's base station. In addition, applying coarse beamforming may only allow low modulation and coding scheme (MCS) for transmission. As such, after establishing an initial connection, the network has to spend additional resources on performing beam refinement to find one or more suitable fine beams for data transmission and reception.
As the 3rd Generation Partnership Project (3GPP) is RAN2 Working Group has made an agreement to support RACH procedure during handover, which is an event in which a network transfers an ongoing data session from a source base station to a target base station, there is a need in the art for beam refinement during the handover as a UE moves across the boundary of a source cell and a target cell, such that the quality-of-service (e.g., a high speed and/or high volume data communication from and/or to the UE) can be maintained during and immediately after the handover.